Typically, synthetic rope or cordage is made up of many thousands of individual strands of synthetic fiber. Each of these fibers has a certain load-bearing capacity (e.g., breaking strength), and theoretically, the total load-bearing capacity of the rope should be equal to the sum of these. However, in practice, this is not so: during the normal fabrication of the rope, the individual fibers do not all end up being of equal length, and so some of these take up the load while others may not do so until the shorter strands break. This problem has become more severe with the advent of ultrahigh strength fibers which stretch very little (e.g., 2%) before parting, as opposed to the 40% stretch or so which was exhibited by earlier nylon fibers and the like.
As a result, the actual load-bearing capacity of a rope is normally some relatively small fraction of the combined capacity of its fibers. In the art, this is expressed as "translational efficiency". For example, relatively large diameter synthetic lines (e.g., 3/8"-4") may typically have a translational efficiency as low as 30-40%. As a result, for a given application, these ropes must be much larger, heavier, and more difficult to handle than would be the case if their translational efficiencies were nearer their theoretical maximums.
Heat stretching of synthetic lines can dramatically increase translational efficiency. When the line is heated, the modulus of elasticity of the fibers is reduced, and then when tension is applied, the shorter fibers are stretched out until the longer ones begin to take a load, and are also stretched out; finally the great majority of the fibers will have the same length and so will be able to bear loads equally.
Heat stretching also tends to improve the structure of synthetic lines on a molecular level. As is known, the molecules of the initial fiber material are often poorly aligned in a somewhat isotropic state; heat stretching essentially "pulls" the polymer material out so as to cause alignment of a greater proportion of the chains of macro molecules along the fiber axis, so that these can bear tensile loading in a more efficient manner.
Heat stretching has been employed previously to achieve these goals, but only with individual yarns or very small diameter synthetic line. For example, both fishing line and bow strings have been successfully stretched by means of a hot gas process. This involves running the line between unequal diameter (or unequal speed) payout and takeup reels, and through a stream of heated air or other gas. The temperature of the gas is typically such that the line would be destroyed if it were to pause in the stream, but the reels are operated at a high rate of speed so that the line is only momentarily softened and stretched in the heated zone before cooling again.
The heat stretch process described in the preceding paragraph works well with very small diameter (e.g., 1/32") synthetic line, but it is inherently unsuitable for use with much larger lines such as braided or twisted rope, which may range upwardly of 4" in diameter. Firstly, the arrangement of high speed payout and takeup reels is simply impractical for handling of rope of this size, at least on an economical basis. Also, the insulating qualities of the rope material would tend to prevent the core of the rope from becoming sufficiently heated to permit stretching before the exterior of the rope degraded in the hot gas stream; the heated gas provides a poor medium for uniformly heating the rope material, and it is also very difficult to control this so as to maintain an accurate temperature very close to the melting point of the rope fibers.
Yet another serious problem stems from the weight of the rope itself (e.g., up to 5 lbs./ft. or more). If a segment of the rope is suspended between a pair of support points (for example, between a pair of eyes, or between a payout and takeup reel), this weight will tend to make the segment droop downwardly towards its center and place a heavy strain on the rope near the support points; if the rope has been heated for stretching, this will tend to cause the material to over-stretch and "neck down" near the support points, with a result that the utility of the rope is destroyed.
While these problems have previously presented themselves with respect to heat stretching synthetic ropes, certain newly developed fiber materials exhibit characteristics which intensify these difficulties. One such a material is an ultrahigh molecular weight polyethylene (UHMWPE) fiber marketed by Allied Signal Corporation under the trademark "SPECTRA". This is a high specific strength material which is very abrasion and UV resistant, and which possesses a high specific modulus of elasticity and a low specific gravity. These qualities render it highly desirable for use in rope. However, the material also presents severe difficulties from the standpoint of previously-known heat stretching techniques: it possesses a low melting point (147.degree. C.) and its tensile properties drop off rapidly near this temperature, and it also acts as an excellent thermal insulator. Accordingly, these characteristics make it very difficult to stretch a rope made of the SPECTRA.TM. fibers using conventional heat stretch processes.